Two sources with knowledge of Kelly's decision say the continual bad press about his history in banking — per the NYT, he was barred from working in the finance industry because of a banking violation — made Kelly decide "enough is enough."

Quoted: Pruitt, in a statement, praised Kelly's work.

“Kell Kelly’s service at EPA will be sorely missed. In just over a year he has made a tremendous impact on EPA’s Superfund program, serving as chair of the Superfund Task Force and presiding over the development of the steps necessary to implement the recommendations in the report," he said, referring to a set of recommendations on bolstering cleanups issued last year.

"Kell has made a point to visit dozens of Superfund sites across the country and has met directly with impacted citizens, community groups and responsible parties," Pruitt said.

Big picture: The resignation comes as Pruitt is embroiled in a series of controversies around his travel and security spending, raises for top aides and more.

While many senior officials at the White House want Pruitt gone — one told Axios his firing is inevitable and part of the need to "rip off the band-aid" — President Trump continues to stick by him.

One level deeper: Kelly is a former banking executive in Oklahoma but was banned last year from the industry by the Federal Deposit Insurance Corporation. He "did not admit or deny the agency's allegations," the Oklahoman reported last year.

However, in response to a question about his history in the industry, an EPA spokesman emailed this quote in his defense . . .

“Kell Kelly is a man of high integrity. During my time as CEO of the American Bankers Association, Mr. Kelly served as my chairman and helped lead the association through a difficult period following 2008 financial crisis; Administrator Pruitt is fortunate to have him,”said Frank Keating, former Oklahoma governor and former CEO of the American Bankers Association.